1. Field of the Invention
The invention relates to image forming apparatuses, such as printers, copying machines, facsimile machines, etc., which record images on a sheet of paper using an electrophotographic process. The invention is particularly advantageous in the context of small image forming apparatuses having a heat type fixing device, however, various features of the present invention may also be advantageously utilized in a wide variety of image forming apparatuses.
2. Discussion of Background
Image forming apparatuses, such as printers, copying machines, facsimile machines, etc., must meet a number of requirements, such as a fast printing speed, high printing image quality, and often the requirement for color printing. In addition, there is currently a great demand for image forming apparatuses which are suitable for personal use. In the personal use market, providing an image forming apparatus which is compact and low in cost is of the utmost importance. However, providing a compact apparatus requires a high density disposition of the various components within the apparatus, which can lead to several problems.
One of the problems in providing a compact image forming apparatus is that a heat type fixing device can be in close proximity to an image carrier or photosensitive body, which is utilized in forming an image on the paper. With the heat type fixing device installed in close proximity to the photosensitive body, water/moisture in the paper can evaporate from the paper, such that moist air is present in the region of the photosensitive body. Starchy materials of the paper may also be present in the air in the region of the photosensitive body, and such undesirable substances can adhere to the photosensitive body. As a result, the image quality can deteriorate, for example, image portions which should be black become white in the final image formed on the paper, or vice versa.
FIGS. 12 and 13 show a small-size laser printer as an example of a small-size image forming apparatus. As shown in FIG. 12, numeral 10 designates the overall structure of the image forming apparatus, including an upper structure 10a, including, for example, a lid, and a lower structure 10b. The upper structure is mounted upon the lower structure by a shaft 11 to allow convenient opening and closing. A photosensitive body 12 is installed at approximately the center of the structure 10, with the various electrophotographic processing devices disposed about the photosensitive body, including a charger 13, developing device 14, transfer device 15 and cleaner 16. The photosensitive body rotates in the direction shown by the arrow.
A paper supply cassette is typically provided above the electrophotographic devices, with an optical writing device 18 installed below the electrophotographic devices. The paper is conveyed from the paper supply cassette by a paper feed roller 20, and is conveyed to the upper side of the photosensitive body 12 in properly timed relation utilizing a pair of resist rollers 21. As the photosensitive body rotates in the direction of the arrow, the surface of the photosensitive body is uniformly charged by the charger 13, and thereafter, an electrostatic latent image is formed on the photosensitive body 12 by light. This latent image is then realized or visualized by toner as the photosensitive body passes through the developing device 14. A transfer device then transfers the visualized image from the photosensitive body to a lower face of a sheet as the sheet of paper is conveyed in contact with the photosensitive body.
The sheet having the image formed thereon is then guided by a conveying guide 23, and then between a fixing roller 17a and pressurizing roller 17b which constitute a fixing device 17. The fixing device 17 fixes the visualized or developed image upon the sheet of paper. The sheet of paper from the fixing device 17 is then conveyed to a paper discharging section via a paper discharging roller. The conveyed sheet of paper is guided by the paper discharging guide 26, and then stacked upon a paper discharge tray 27.
An exhaust fan is typically installed in a left, back side of the image forming apparatus in relation to the operating side of the apparatus (i.e., with the operating side being at the right side of the apparatus shown in FIG. 12). FIG. 13 clearly illustrates a back, left location of a fan 50. The exhaust fan typically is near the fixing device 17 and charger 13, such that excessive operating temperatures (which may be caused by the fixing device) are avoided, and also such that any ozone (which may be attributable to the charger 13) is dissipated. However, in accordance with one aspect of the present invention, it has been recognized that typical exhaust system arrangements suffer from a number of performance shortcomings. Typically, during operation of the fan, air flow occurs about the photosensitive body near the fixing device as shown by the arrows in FIGS. 14 and 15. The fan is installed in the left, back side, and below the fixing device 17 as shown in FIGS. 12 and 13, such that it does not interfere with other components of the image forming apparatus. The air, typically including moisture, at the paper insert entrance thus flows down between the photosensitive body and the fixing device 17 as shown by the streamline A' in FIG. 14. As shown more specifically in FIG. 15, the air flows toward the underside of the fixing device and around the side opposite to that of the fan as shown at A in FIG. 15. Closer to the fan, the flow occurs in more of a horizontal direction as shown at B. Still closer to the fan, and as shown at C, the flows A and B are mixed, however, a regular flow is not produced, and portions of the air can thus accumulate, at least temporarily, in the region shown at C in FIG. 15. As a result, the image quality can deteriorate, and, in fact, such problems (i.e. poor image quality) have been recognized more frequently in the left side of the machine with reference to the operating position as described above.
Another problem caused by insufficient exhaust, relates to toner which is floating in the region between the photosensitive body and the fixing device, such as toner which has not adhered to the paper or the photosensitive body at a desired location. The floating toner can then adhere to a part of the paper or the photosensitive body at an undesirable location, thus lowering the print quality. The occurrence of such problems more frequently in the left side can also demonstrate the relationship of this problem to the ineffective exhaust.
Moreover, with a small-size image forming apparatus, for example as shown in FIG. 12, the fixing device 17 is closely adjacent to the photosensitive body 12, such that as a sheet of paper is being conveyed, the sheet can simultaneously contact part of the fixing roller 17a and part of the photosensitive body 12. The sheet can thus bridge across the gap between the photosensitive body and the fixing device, as shown in FIG. 12. As a result, air in the region between the photosensitive body and the fixing device, which may include moisture or other undesirable substances, cannot escape upwardly, and thus tends to remain in the region defined between the photosensitive body 12, the paper, and the fixing device 17.
Moreover, since the sheet of paper is conveyed on the upper side of the photosensitive body 12, the image is transferred to the lower side of the paper, and as the paper is conveyed between the transfer device 15 and the fixing device 17, toner at the underside of the paper can fall from the paper. Disassociation of the toner from paper can be caused by even very minute vibrations, often vibrations associated with conveyance of the paper.
Thus, problems associated with improper exhaust can be particularly severe in small-size image forming apparatuses. One solution to such problems could be the installation of a plurality of fans in order to make the exhaustion of air more complete. However, the provision for a plurality of fans can increase the cost and size of the image forming apparatus.
Accordingly, an improved exhaust flow arrangement is needed for image forming apparatuses. Such an exhaust flow system should be capable of providing a reliable and complete exhaustion of air, particularly air located in the region between the photosensitive body and the fixing device of an image forming apparatus in order to avoid deterioration in print quality. Moreover, the exhaust system should be suitable for use in a small size image forming apparatus, without increasing the size of the image forming apparatus.